How can I pause a render in progress and then resume it at a later time?
On Mac OS it is not necessary to have started Blender from the command line.  You may use the "kill" command to send a stop and continue signal...

$ ps -ax | grep Blender
3554 ??       750:42.18 /.../.../Contents/MacOS/Blender
$ kill -s SIGSTOP 3554
$ kill -s SIGCONT 3554




There is a way to 'pause' Blender (or any software for that matter), but it relies on the operating system rather than Blender itself.

Mac OS X and Linux:


You must start Blender from the command line or terminal.
At any time, you can completely suspend Blender by pressing Ctrl+Z from the terminal window.
The terminal should say something like:

^Z
[1]+  Stopped                 /Applications/blender.app/Contents/MacOS/blender

You cannot interact with it until you you type fg in the terminal. Which will bring Blender back to the foreground.


Windows:


Have the console window open (Window > Toggle System Console)
Use the cursor to highlight any of the text in this window will freeze the console and pause Blender. This is a super hacky way to actually pause Blender, but it works in a pinch. See https://stackoverflow.com/questions/30418886/how-and-why-does-quickedit-mode-in-command-prompt-freeze-applications for detailed explanation of why this happens.